By combining the Extreme Value Theorem and Fermat's Theorem, we
get
Rolle's
Theorem: Let $f$ be a function that is continuous on a
closed interval $[a,b]$ and differentiable on the open
interval $(a,b)$, and suppose that $f(a)=f(b)=0$. Then there
is a point $x=c$, somewhere between $x=a$ and $x=b$, such
that $f'(c)=0$.
Roughly speaking, the Extreme Value Theorem says that there has
to be a maximum and a minimum. If it's in the interior, Fermat's
Theorem says that it has to be a critical point. Since $f$ is
differentiable on the interior, it has to be a point where
$f'(c)=0$. The details of this argument, as well as the cases
where the maxima and minima are at the endpoints, are found in the
following video: